Device and method for reducing the hydrogen peroxide and peracetic acid content in a water flow

ABSTRACT

A device comprising a measuring device for determining the flow rate of a water stream, a measuring device for determining the concentration of hydrogen peroxide in the water stream, a measuring device for determining the concentration of peracetic acid in the water stream, a metering device for metering a reducing agent into the water stream downstream of the measuring devices for the concentrations, and a control device which, from the flow rate of the water stream, the concentration of hydrogen peroxide in the water stream and the concentration of peracetic acid in the water stream, calculates an amount of reducing agent for reducing the content of hydrogen peroxide and peracetic acid to a desired value and actuates the metering device for metering the reducing agent, makes possible reliable reduction of the content of hydrogen peroxide and peracetic acid in a water stream. The device is suitable for reducing the content of hydrogen peroxide and peracetic acid in a water stream which is withdrawn from ballast water tanks of a ship.

The invention relates to a device and a method for reducing the contentof hydrogen peroxide and peracetic acid in a water stream, in particularin a water stream which is withdrawn from ballast water tanks of a ship.

Peracetic acid is a biocide which has a number of advantages comparedwith other biocides. Peracetic acid, even at low concentrations of lessthan 5 ppm, exhibits a broad biocidal activity against bacteria,phytoplankton and zooplankton, without resistances occurring. Incontrast to most other biocides, peracetic acid in dilute aqueoussolutions is rapidly degraded by hydrolysis and decomposition tosubstances which are no longer biocidally active. In contrast to ozoneor chlorine dioxide, peracetic acid can safely be transported and storedin the form of equilibrium peracetic acid. A treatment of water streamswith peracetic acid, in contrast to chlorine or hypochlorite, does notlead, or leads only to a small extent, to the formation of halogenatedorganic compounds and therefore does not lead to an increase in the AOXcontent. Peracetic acid is therefore suitable for the biocidal treatmentof water streams which are released into the surroundings in a largeamount after the treatment, such as, for example, cooling water streamsor sewage treatment plant discharges and, in particular, ballast waterof ships. The treatment of ballast water with peracetic acid in theSEDNA® method is approved by the International Maritime Organization(IMO) for the removal of phytoplankton and zooplankton.

Although peracetic acid and the hydrogen peroxide that is present inequilibrium peracetic acid due to the production process rapidly degradein the treated water, it can be necessary in some applications, inparticular in the treatment of ballast water, to remove any residualamounts of peracetic acid and hydrogen peroxide still present after thetreatment before the treated water is released to the surroundings.

For removing chloramines or bromamines from treated ballast water, WO02/072478 proposed to add a reducing agent such as sodium thiosulphateor sodium sulphite in a molar excess to the treated ballast water. Inthis method, however, after reducing the chloramine or bromamine, oxygenmust further be introduced into the treated water before it can bereleased to the surroundings.

WO 2004/054932 proposes to add a solution of sodium thiosulphate to theballast water and control this metering via the redox potential of thechlorine-containing ballast water ror removing electrolyticallygenerated chlorine from treated ballast water.

WO 2006/058261 and WO 2008/153808 propose to add a solution of sodiumsulphite to the ballast water and control this metering via a sulphiteanalyser, which releases SO₂ by acid addition and determines this with asensor, in such a manner that the treated ballast water contains excesssodium sulphite for removing electrolytically generated hypochloritefrom treated ballast water.

US 2010/072144 proposes to add a solution of sodium sulphite to theballast water and control this metering via measurement of the redoxpotential in the ballast water after addition of the sodium sulphitesolution in such a manner that the redox potential is in the range from200 to 500 mV for removing hypochlorite from treated ballast water.

U.S. Pat. No. 7,776,224 proposes to measure the concentration ofhydrogen peroxide in the ballast water and add a reducing agent on thebasis of the measured value for removing hydrogen peroxide from treatedballast water. It is further proposed to check after addition of thereducing agent with a hydrogen peroxide densitometer or measurement ofthe redox potential whether unreacted hydrogen peroxide is present.

EP 1 671 932 proposes to add one of the substances iron(II) sulphate,iodide or catalase with the oxidizing agent for a treatment of ballastwater with hydrogen peroxide or equilibrium peracetic acid, in order toachieve decomposition of hydrogen peroxide during the ballast watertreatment.

However, there is still a need for a device and a method that allows toreduce the content of hydrogen peroxide and peracetic acid in a waterstream as required and with which the water stream, after removal ofhydrogen peroxide and peracetic acid, contains no substances hazardousto water.

The inventors of the present invention have established that the methodknown from US 2010/072144 for removing hypochlorite is not suitable forremoval of hydrogen peroxide from a water stream, since, by measuringthe redox potential in the water after addition of a reducing agent, itcannot be reliably established that the water does not contain eitherunreacted hydrogen peroxide or excess reducing agent. Also, the methodknown from WO 2004/054932 for removing chlorine is not suitable forremoving peracetic acid and hydrogen peroxide from a water stream, sincethe amount of reducing agent which would be required for removingperacetic acid and hydrogen peroxide cannot be calculated in advancefrom the redox potential of a water stream containing peracetic acid andhydrogen peroxide.

The inventors of the present invention have therefore developed a deviceand a method by means of which the contents of hydrogen peroxide andperacetic acid can be reliably reduced in a water stream.

The invention relates to a device for reducing the content of hydrogenperoxide and peracetic acid in a water stream (1), comprising a firstmeasuring device (2) for determining the flow rate of the water stream,a second measuring device (3) for determining the concentration ofhydrogen peroxide in the water stream, a third measuring device (4) fordetermining the concentration of peracetic acid in the water stream, ametering device (5) for metering a reducing agent into the water streamdownstream of the second and third measuring devices and a controldevice (6) which, from the flow rate of the water stream, theconcentration of hydrogen peroxide in the water stream, and theconcentration of peracetic acid in the water stream, calculates anamount of reducing agent for reducing the content of hydrogen peroxideand peracetic acid to a desired value and actuates the metering devicefor metering the reducing agent.

The invention additionally relates to a method for reducing the contentof hydrogen peroxide and peracetic acid in a water stream, comprisingmetering a liquid reducing agent into the water stream using a deviceaccording to the invention. The water stream is preferably withdrawnfrom ballast water tanks (10) of a ship.

FIG. 1 shows a device according to the invention in an embodiment havingan additional measuring device (7) for determining the salinity and anarrangement of the second and the third measuring device in a sidestream (9).

The device according to the invention comprises a first measuring device(2) for determining the flow rate of the water stream (1). For thispurpose, measuring devices which which determine a mass flow rate aswell as measuring devices which determine a volumetric flow rate areboth suitable. For the device according to the invention, all measuringdevices known from the prior art for determining the flow rate of awater stream may be used such as, for example, mass flow meters,differential pressure measurements at orifice plates and inductive flowmeters. Preferably, a mass flow meter is used for determining the flowrate of the water stream in order to determine the flow rate of thewater stream reliably even for water streams having different saltcontents.

The device according to the invention additionally comprises a secondmeasuring device (3) for determining the concentration of hydrogenperoxide in the water stream (1). Suitable measuring devices are allthose known from the prior art with which the concentration of hydrogenperoxide may be determined in water and which do not exhibit, or exhibitonly to a slight extent, a cross-sensitivity to peracetic acid. Suitablemeasuring devices are, for example, those which determine theconcentration of hydrogen peroxide colorimetrically and use a colourreaction specific for hydrogen peroxide such as, for example, thereaction of hydrogen peroxide with titanyl sulphate, forming a solubletitanium(IV) peroxo complex. Preferably, an amperometric sensor is usedfor determining the concentration of hydrogen peroxide, particularlypreferably an amperometric sensor at which an oxidation of hydrogenperoxide proceeds according to the reaction equation

H₂O₂→O₂+2 H⁺+2 e⁻.

Suitable amperometric sensors for hydrogen peroxide that do not exhibitcross-sensitivity to peracetic acid are commercially available, forexample from ProMinent® under the name DULCOTEST® PEROX. The responsetime of these sensors can be adapted by the manufacturer by exchangingthe membrane which covers the sensor to the rate of change of thehydrogen peroxide concentration in the water stream that is to betreated.

The device according to the invention further comprises a thirdmeasuring device (4) for determining the concentration of peracetic acidin the water stream (1). Suitable measuring devices are all those knownfrom the prior art with which the concentration of peracetic acid may bedetermined in water and which do not show, or show only to a minorextent, a cross-sensitivity to hydrogen peroxide. Suitable measuringdevices are, for example, those which determine the concentration ofperacetic acid colorimetrically and use a colour reaction specific toperacetic acid, such as, for example, the reaction of peracetic acidwith 2,2-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) diammoniumsalt (ABTS), forming a soluble dye. Preferably, an amperometric sensoris used for determining the concentration of peracetic acid,particularly preferably an amperometric sensor at which a reduction ofperacetic acid proceeds according to the reaction equation

CH₃COOOH+2 H⁺+2 e⁻→CH₃COOH+H₂O.

Suitable amperometric sensors for peracetic acid which show asufficiently low cross-sensitivity to hydrogen peroxide are commerciallyavailable, for example from ProMinent® under the name DULCOTEST® PAA.The response time of these sensors can be adapted by the manufacturer,by exchanging the membrane which covers the sensor, to the rate ofchange of peracetic acid concentration in the water stream that is to betreated. Amperometric sensors that are likewise suitable arecommercially available amperometric sensors for determining the totalchlorine content, for example the sensors marketed by ProMinent® underthe name DULCOTEST® CTE-1. Since, due to the rapid reaction of chlorineand hypochlorite with hydrogen peroxide, a water stream containinghydrogen peroxide can contain only small amounts of chlorine andhypochlorite, and the amperometric sensors for determining the totalchlorine content also determine peracetic acid with lowcross-sensitivity to hydrogen peroxide, the content of peracetic acidcan also be reliably determined in the water stream using such sensors.

The use of amperometric sensors for determining the concentrations ofhydrogen peroxide and peracetic acid makes possible a substantiallyautomated operation of the device according to the invention by staffsuch as, for example, a ship's crew, that has no training in operatinganalytical equipment.

Instead of two separate measuring devices for determining theconcentrations of hydrogen peroxide and peracetic acid, it is alsopossible to use one measuring device, which determines both theconcentration of hydrogen peroxide and the concentration of peraceticacid, in the device according to the invention. One example of such ameasuring device is an automated titration with sequential cerimetricdetermination of the hydrogen peroxide concentration and iodometricdetermination of the peracetic acid concentration.

The measuring devices for determining the concentrations of hydrogenperoxide and peracetic acid are preferably arranged in a side stream (9)of the water stream in order to avoid damage to the measuring devices bysolids carried by the water stream. For the same purpose, preferably afilter is arranged in the side stream upstream of the measuring devices.

The device according to the invention additionally comprises a meteringdevice (5) for metering a reducing agent into the water stream (1)downstream of the second and third measuring devices. Suitable meteringdevices are those for continuous or intermittent metering of a reducingagent, which reducing agent is preferably gaseous or liquid, andparticularly preferably liquid. Preferably, the metering devicecomprises a storage vessel (8) and a controllable metering pump (5) forliquid reducing agent, such that a continuous metering of the liquidreducing agent is possible with a variable volumetric flow rate.Particularly preferably, the metering device comprises apositive-displacement metering pump such as, for example, a diaphragmpump, gear pump or piston pump which makes possible setting a calculatedvolumetric flow rate for metering liquid reducing agent.

The device according to the invention further comprises a control device(6) which calculates an amount of reducing agent for reducing thecontent of hydrogen peroxide and peracetic acid to a desired value fromthe flow rate of the water stream (1), the concentration of hydrogenperoxide in the water stream and the concentration of peracetic acid inthe water stream, and actuates the metering device (5) for metering thereducing agent. The control device can be designed as a hard-wiredcontroller or as a calculation and control program on a process controlcomputer. The calculation of the amount of reducing agent from the flowrate of the water stream, the concentration of hydrogen peroxide in thewater stream and the concentration of peracetic acid in the water streamcan proceed using empirical conversion factors determined by experimentsor, preferably, using conversion factors calculated from thestoichiometry of the reduction reaction. For salt-free water streams anda reduction using an aqueous solution of sodium sulphite, the conversionfactors can be calculated on the basis of the reaction equations (I) and(II).

H₂O₂+Na₂SO₃→H₂O+Na₂SO₄  (I)

CH₃COOOH+Na₂SO₃→CH₃COOH+Na₂SO₄  (II)

For liquid reducing agents which are metered via a positive-displacementmetering pump, the volumetric flow rate to be set at the metering pumpcan be calculated directly from the calculated amount of reducing agentand the metering pump actuated accordingly.

In a preferred embodiment, the device according to the inventioncomprises an additional measuring device (7) for determining thesalinity in the water stream (1). The expression salinity heredesignates the dimensionless salinity S on the Practical Salinity Scale1978. The salinity can be determined on the basis of densitymeasurements, and preferably on the basis of the electrical conductivityusing a conductivity sensor. In this embodiment, the amount of reducingagent is calculated by the control device with the salinity. Preferably,here, the amount of reducing agent calculated for a salt-free waterstream is corrected by a correction factor for the salinity determinedby experiments. For salt-containing water streams and a reduction withan aqueous solution of sodium sulphite, preferably the amount ofreducing agent calculated for a salt-free water stream is increased by afraction proportional to the salinity. Taking into account the salinityin metering the reducing agent makes possible reliable reduction of thecontent of hydrogen peroxide and peracetic acid to below predeterminedlimits even for a variable salt content of the water stream, withoutoverdosing of reducing agent occurring.

In the method according to the invention for reducing the content ofhydrogen peroxide and peracetic acid in a water stream, a liquidreducing agent is metered into the water stream (1) by a deviceaccording to the invention. The water stream is preferably a waterstream treated by adding equilibrium peracetic acid as biocide, inparticular a cooling water stream, or a sewage treatment plantdischarge, and most preferably, a water stream which is withdrawn fromballast water tanks (10) of a ship.

In the method according to the invention, preferably, an aqueoussolution of sodium sulphite is used as liquid reducing agent.

The method according to the invention makes possible a reliablereduction of the content of hydrogen peroxide and peracetic acid in awater stream below predetermined limiting values, wherein, by usingsodium sulphite as reducing agent, the water stream after the treatmentno longer has properties impairing the water quality. This makes itpossible to discharge a ballast water treated with equilibrium peraceticacid for destroying phytoplankton and zooplankton into bodies of watersuch as, for example, constricted port basins in which the ballast wateris diluted only poorly, without impairing the water quality of the bodyof water.

EXAMPLES

For the working exemples embodiments, water which had been taken from adrinking water supply network was treated with 80 ppm equilibriumperacetic acid that contained 14.4% by weight peracetic acid and 13.5%by weight hydrogen peroxide. After the treatment with equilibriumperacetic acid, the water contained 11.9 ppm peracetic acid and 13.3 ppmhydrogen peroxide on a weight basis.

To a stream of the water that had been treated with equilibriumperacetic acid, an aqueous solution of sodium sulphite was meteredcontinuously in a device according to the invention as per FIG. 1. Theconcentrations of hydrogen peroxide and peracetic acid were determinedhere using amperometric sensors from ProMinent®. In Example 1, 1.03times the stoichiometric amount of sodium sulphite, calculated accordingto reaction equations (I) and (II) from the concentrations of hydrogenperoxide and peracetic acid and the flow rate of water, was metered. InExample 2, 1.21 times the calculated stoichiometric amount of sodiumsulphite was metered.

In Example 1, the water contained 0.1 ppm peracetic acid and 1.0 ppmhydrogen peroxide, on a weight basis, after metering sodium sulphite. InExample 2, the water contained 0.2 ppm peracetic acid and 0.1 ppmhydrogen peroxide, on a weight basis, after the metering of sodiumsulphite.

For the water treated with equilibrium peracetic acid and the waterstreams obtained in Examples 1 and 2, the inhibition of algal growth andthe acute immobilization of daphnia were determined in accordance withOECD guidelines 201 and 202 for the testing of chemicals.

The water treated with equilibrium peracetic acid caused, undiluted,complete inhibition of the algal growth of Desmodesmus subspicatus withan EC₅₀ value of 46% for the inhibition of the growth rate and 25% forthe inhibition of the yield. In contrast, the water stream obtained in

Example 1 caused, undiluted, only a statistically insignificantinhibition of the growth rate of 5%. The water stream obtained inExample 2 caused, undiluted, an inhibition of the growth rate of 13%.

The water treated with equilibrium peracetic acid caused, undiluted,complete immobilization of Daphnia magna with an EC₅₀ value of 12%. Incontrast, the water streams obtained in Examples 1 and 2 caused, evenundiluted, no immobilization and displayed no recognizable effect ondaphnia.

The examples show that, with the device according to the invention andthe method according to the invention, the contents of hydrogen peroxideand peracetic acid may be reliably reduced in a water stream containinghydrogen peroxide and peracetic acid in such a manner that uponintroduction into bodies of water the water stream has no harmfuleffects on water organisms.

LIST OF REFERENCE NUMERALS

-   (1) water stream-   (2) measuring device for determining the flow rate-   (3) measuring device for determining the concentration of hydrogen    peroxide-   (4) measuring device for determining the concentration of peracetic    acid-   (5) metering device for metering a reducing agent-   (6) control device-   (7) measuring device for determining salinity-   (8) storage vessel for liquid reducing agent-   (9) side stream-   (10) ballast water tanks

1-10. (canceled)
 11. A device for reducing the content of hydrogen peroxide and peracetic acid in a water stream, comprising: a) a first measuring device for determining the flow rate of said water stream; b) a second measuring device for determining the concentration of hydrogen peroxide in said water stream; c) a third measuring device for determining the concentration of peracetic acid in said water stream; d) an additional measuring device for determining the salinity in said water stream; e) a metering device for metering a reducing agent into said water stream downstream of the second and third measuring devices; and f) a control device which, from the flow rate of said water stream, the concentration of hydrogen peroxide in said water stream, the concentration of peracetic acid in said water stream and the salinity, calculates an amount of reducing agent for reducing the content of hydrogen peroxide and peracetic acid to a desired value and actuates the metering device for metering the reducing agent.
 12. The device of claim 11, wherein the measuring device for determining the concentration of hydrogen peroxide in said water stream comprises an amperometric sensor.
 13. The device of claim 11, wherein the measuring device for determining the concentration of peracetic acid in said water stream comprises an amperometric sensor.
 14. The device of claim 11, wherein the measuring device for determining the salinity comprises a conductivity sensor.
 15. The device of claim 11, wherein the metering device for metering a reducing agent comprises a storage vessel and a controllable metering pump for liquid reducing agent.
 16. The device of claim 11, wherein the second and third measuring devices are arranged in a side stream of said water stream.
 17. A method for reducing the content of hydrogen peroxide and peracetic acid in a water stream, comprising metering a liquid reducing agent into the water stream using the device of claim
 11. 18. The method of claim 17, wherein said water stream is withdrawn from ballast water tanks of a ship.
 19. The method of claim 17, wherein the liquid reducing agent is an aqueous solution of sodium sulphite. 